1. Field of the Invention
The invention relates to a method and an apparatus for controlling physical properties of a rod made of smokable material.
2. Description of the Prior Art
Although similar problems arise in other smokable articles as well hereinafter these problems will be discussed only in conjunction with the manufacture of cigarettes.
In the manufacture of cigarettes various techniques are used to permit continuous control of the physical properties of the rod of tobacco fibres during the cigarette production and thus finally of the corresponding properties of the finished cigarette. It is,for example known to determine the density of the rod essential to the weight constancy of the cigarette and corresponding in turn to its weight, to compare the actual value determined with a desired value and to control the formation of the rod in dependence upon the result of this comparison (German Offenlegungsschrift No. 2 208 944). For measuring the density for example the absorption of radiation in the rod of tobacco fibres may be utilized, in particular the absorption of beta rays or microwaves.
German Offenlegungsschrift No. 2 728 797 also describes a method of making cigarette filters in which the resistance to draw of the filter tow is determined, compared with a desired value and the result of the comparison used for the filter tow supply; the tow supply is controlled so that the pressure drop across the measured tow remains substantially constant.
A further method is disclosed in the published European patent application No. 32 399, the moisture content, the mass, the machine speed and the hardness of a rod of tobacco fibres being determined for the control; the measured hardness value is corrected by means of the other parameters, i.e. moisture content, mass and machine speed, thereby giving a correcte actual value which is compared with a desired value. Depending on the result of this comparison the ecreteur (trimmer discs) is adjusted and set at a predetermined height of the tobacco rod in the formation of the latter.
Sensors suitable for detecting the hardness of the tobacco rod are disclosed in German Offenlegungsschrift No. 3 306 543 and German Offenlegungsschrift No. 3 404 635.
The method hitherto usual in practice of weight/density control has, however, the following disadvantages: on changes of the tobacco filling capacity large deviations of the resistance to draw or the hardness of the tobacco rod may occur leading to end dropout; large deviations in the resistance to draw also have an unfavourable effect on the drawability, fullness of flavour and character as well as the degree of ventilation of the finished cigarette. The hardness and the resulting end dropout of a cigarette are, however, important quality features to the consumer.
The regulation of the hardness, possibly also taking account the moisture, the mass and the machine speed, also leads, however, to problems because,although an important quality feature can be kept constant; the other physical properties, in particular density/weight and resistance to draw, are no longer taken into account. Thus, once again great fluctuations can occur which have an unfavourable effect on the quality of the finished cigarette.
Moreover, further problems result due to the use of expanded and reconstituted tobacco as is employed to an increasing extent. For when mixture fluctuations occur the sensor for the density or the weigth of the tobacco rod "detects" too little tobacco, i.e. proportionally the rod contains too much expanded tobacco in its mixture so that the actuator for the weight control, generally the ecreteur (trimmer discs) is adjusted in the sense of enlarging the rod as a result more tobacco enters the cigarette. This, however, increases the resistance to draw and the hardness of the cigarettes and thus quality features which are important to the consumer; as a rule, the consumer will notice changes in the resistance to draw and hardness sooner than small fluctuations in the weight of the cigarette.